1. Field of the Invention
The present invention relates to produce containers made from expandable polymer particles, e.g. expandable polystyrene particles (EPS), and to methods for improving the resistance of fumigant, e.g. methyl bromide, sorption in the produce containers in a fumigation process.
2. Background Art
Fumigants are used to eradicate pests from containers for the international shipment of produce, e.g. fruits, e.g. grapes, and vegetables, e.g. tomatoes. In general, the fumigation process is necessary when produce is either imported or exported in order to control the infestations of pests, e.g. Mediterranean fruit fly. A commonly used fumigant is methyl bromide, which is highly toxic.
The types of containers most commonly used for shipping produce are made of wood, paperboard, or plastic materials, e.g. expandable polymer particles, e.g. expandable polystyrene particles. Wooden containers need to be extremely inexpensive and yet sturdy and the cost of wood has become exorbitant. Cardboard or paperboard containers tend to collapse when a number of filled containers are stacked on top of each other. A container made of expandable polystyrene particles provides the sturdiness needed to support the number of filled containers when stacked on top of each other and is relatively inexpensive compared to wooden containers. Examples of containers made from expandable polystyrene particles in a molding process are disclosed in U.S. Pat. No. 5,016,777 issued to Morley Marvin on May 21, 1991 and in U.S. Pat. No. 5,690,272 issued to James England on Nov. 25, 1997. The latter patent discloses a container that is hand assembled and is suitable for shipping produce.
However, due to the nature of foamed polymer, particularly foamed polystyrene, in that it contains open cells or voids, produce containers made of these particles have a relatively high level of fumigant absorption rate compared to produce containers made of wood or paperboard. Additionally, the fumigant tends to remain desorbed in the walls of the container made of expandable polymer particles for a long period of time after the fumigation process.
When produce is imported into or exported from a country, the general procedure is to stack two or more rows of containers containing the vegetables or fruits onto a pallet, and then transport the pallet into a fumigation chamber. The fumigant, which typically is methyl bromide, is delivered into the chamber for two hours in order to kill the infestation that may be present, and then to interrupt the delivery of the fumigant for the next two hours so that the methyl bromide can dissipate before the chamber can be safely opened to remove the pallet and containers.
The United States Department of Agriculture (USDA) requires that the fumigation process meet the following criteria: 1) The methyl bromide concentration in the fumigation chamber during the fumigation process should not be lower than the minimum concentration required by USDA schedule T101-I-2-1, e.g. 48 ounces/1000 feet3 for the first half hour and 38 ounces/1000 feet3 for 2.0 hours, at a temperature between 40-49° F. (Table 1). 2) The residual methyl bromide concentration should not be more than 5 ppm after degassing, i.e. removing the methyl bromide from the chamber.
Studies have shown that current produce containers made of expandable polystyrene particles cannot maintain the mandated minimum methyl bromide concentrations during the fumigation process when the initial concentration required by USDA, e.g. 64 ounces/1000 feet3 at 40° to 49° F., is applied because these containers absorb a great amount of methyl bromide. Data also show that these containers have a high residual concentration (higher than the threshold limit value (TLV) of 5 ppm) after a typical aeration process following the fumigation process.
Therefore, the produce market requires improved expandable polymer containers that absorb and retain less fumigant during the fumigation process.
There is also a need to provide a method for improving the resistance of fumigant, e.g. methyl bromide, sorption in a produce container made from expandable polymer particles.